Outline:
3/12/07
Chem. Dept. Seminar Wed @ 4pm
 2 more lectures until Exam 2…
 Chemistry Advising – Today @ 4pm

Today:
More Chapter 18
Polyprotic acid titrations
Solubility Product (Ksp)

Worksheet #8 practice…
#1a.
pH = 2.5
[H2A] = 0.997 M
[HA-] = 0.00316 M
[A2-] = 1  10-8 M
#1b.
#2a.
pH = 10.5
pH = 9.74
#2b.
pH = 11.1
Quiz # 7
Please put away all books/papers
If you don’t have a calculator,
just set up the problems fully…
Quiz # 7
Please turn your papers over and
pass them to the right…
Quiz #7 : Buffers
#1
pH = pKa + log ([base]/[acid])
4.00 = 4.75 + log ([base]/[1.0M])
[base] = 10-0.75 = 0.178 M
[base] = 0.178 mol/1.0L
= 14.6 g CH3COONa / 1.0L
Quiz #7 : Weak Base
#2
Pyr + H2O = pyrH+ + OH0.015-x
+x
+x
Kb = 10-8.72 = 1.9110-9
1.9110-9 = x2 / 0.015
x = 5.34 10-6 = [OH-]
pH = 14 – log( 5.34 10-6) = 8.73
Quiz #7 : Titration
#3
HA + OH-  A- + H2O (titration)
0.0025 + 0.0005  0 + 0
0.0020 + 0
 0.0005
(init)
(equil)
pH = pKa + log ([base]/[acid])
pH = 3.74 + log ([0.0005]/[0.0020])
= 3.14
Titration of Polyprotic Acids
Weak acid:
Ka1 = x2/[H2A]
Titration of Polyprotic Acids
Buffer:
pKa2+log[A]/[HA]
Buffer:
pKa1+log[HA]/[H2A]
Titration of Polyprotic Acids
log((Ka1× Ka2)0.5)
See page 785
Try example 18 – 12 (page 787)
Sulfurous Acid, H2SO3, has two acidic
hydrogen atoms, with pKa values of 1.85
and 7.20. Construct a titration curve for
the titration of 125 mL of 0.150 M
sulfurous acid with 0.800 M NaOH.

changes color as the titration passes the
stoichiometric point if :
pKin≈ pHstoichiometric point
The Solubility-Product Constant, Ksp
• Consider
BaSO4(s)
• for which
•
Ba2+(aq) + SO42-(aq)
K sp  [Ba 2 ][SO24- ]
Ksp is the “solubility product”.
(BaSO4 is ignored because it is a pure
solid so its concentration is constant.)
Solubility Equilibria
• The solubility product is another
example of equilibrium calculations
• Solubility product calcs depend on
the common ion effect (LeChâtelier).
• They have particular applications
with metal ions and pH calculations
(environmental applications).
Types of Equilibrium Constants:
Lots of different names….
Keq, KH , Ksp, Ka , Kb, Kf , Kc, Kp…
All the same idea!
Solubility Equilibria

Insoluble compounds: solubility is less
than 0.01 mol of dissolved material per
liter of solution, Ksp << 1

Slightly soluble: 10-5 < Ksp < 10-2

Soluble: Ksp > 10-2
Solubility Equilibria
• The solubility product is another
example of equilibrium calculations
• Solubility product calcs depend on
the common ion effect (LeChâtelier).
• They have particular applications
with metal ions and pH calculations
(environmental applications).
Factors that Affect Solubility
The Common Ion Effect
• Solubility is decreased when a common
ion is added (Le Châtelier again)
CaF2(s)
Ca2+(aq) + 2F-(aq)
• as F- (from NaF, say) is added, the
equilibrium shifts left, therefore CaF2(s)
is formed (precipitation occurs).
• As NaF is added to the system, the
solubility of CaF2 decreases.